As known by those having ordinary skill in the art, chemically, most of the rare earth (RE) elements (atomic numbers 57 through 71) are trivalent. Cerium alone is known to form compounds with a valence of +4, such as CeO2 (ceria). Cerium is believed to be a unique material with regard to the mixed valence states provided, both +3 and +4. Cerium of valence +3 is generally referred to as cerous, while with valence +4 is generally referred to as ceric. Cerium oxide includes both ceric oxide and cerous oxide. Cerous oxide is also known as Cerium III oxide and has the formula Ce2O3. Ceric oxide is known as ceria, cerium dioxide and cerium IV oxide and has the chemical formula CeO2.
Cerium oxide has been used as a catalyst in industrial applications because of its potent redox-active properties, including as a catalyst, to remove carbon monoxide, hydrocarbons and nitric oxide species (NOx) from exhaust gas.5, 6 Nitric oxide is formed, for example, when the nitrogen present in atmospheric air is subjected to high temperatures, such as those normally found in conventional combustion and incineration processes. In addition, nitric oxide is well-known to be formed during wastewater treatment, such as with the use of bacteria, such as Nitrosomonas, which are widely used to eliminate nitrogenous compounds from wastewater. A number of other processes produce nitric oxide at measurable levels, such as certain etching and welding processes. Nitric oxide is rapidly oxidized in air to nitrogen dioxide, which is a major pollutant; therefore, improved methods for its removal and detection are desired.